2001ApJ...560..992M

We show that the Fe XVII I(17.10 Å)/I(17.05 Å) line ratio observed in the Chandra High-Energy Transmission Grating (HETG) spectrum of the intermediate polar EX Hya is significantly smaller than that observed in the Sun or other late-type stars. Using the Livermore X-Ray Spectral Synthesizer, which calculates spectral models of highly charged ions based on HULLAC atomic data, we find that the observed I(17.10 Å)/I(17.05 Å) line ratio can be explained if the plasma density n_e≳3x10¹⁴ cm^–3. However, if photoexcitation is included in the level-population kinetics, the line ratio can be explained for any density if the photoexcitation temperature T_bb≳55 kK. For photoexcitation to dominate the Fe XVII level-population kinetics, the relative size of the hot spot on the white dwarf surface must be f≲2%. This constraint, and the observed X-ray flux, requires a density n≳ 2x10¹⁴ cm^–3 for the postshock flow. Either way, then, the Chandra HETG spectrum of EX Hya requires a plasma density that is orders of magnitude greater than that observed in the Sun or other late-type stars.